1. Field of the Invention
The present invention relates to a ceramic multilayer substrate with an improved connection structure between internal patterns and an external terminal, and a method for manufacturing the substrate, and more particularly to a low temperature co-fired ceramic multilayer substrate formed by vertically stacking and firing a plurality of ceramic sheets or layers, in which a connection bar is vertically formed on connection areas between internal patterns and an external terminal of each ceramic sheet, thereby preventing metallic conductive layers of the internal patterns from being deformed during the formation of the external terminal and stably connecting the internal patterns to the external terminal, and a method for manufacturing the substrate.
2. Description of the Related Art
A technique for manufacturing a low temperature co-fired ceramic (hereinafter, being referred to as “LTCC”) substrate is a process in which an internal electrode and passive elements (R, L, and C) for given circuits are formed on a green sheet made of glass ceramic by a screen printing method using a metal with high electric conductivity such as Ag, Cu, or etc and a plurality of the green sheets are stacked vertically and then fired (generally at less than 1,000° C.) so as to manufacture MCM (multi-chip modules) and multi-chip packages.
Since the ceramic substrate and the metallic elements are co-fired, the LTCC technique can form the passive elements (R, L, and C) within a module, thereby obtaining a complex configuration including many components and being advantageous in terms of miniaturization.
Since the LTCC substrate comprises the embedded passive elements, the LTCC substrate can be formed as a SOP (System-On-Package), thereby minimizing a parasitic effect generated in parts of a SMD (Surface Mounted Device). Further, the LTCC substrate reduces electrical noise generated at soldering parts in surface mounting, thereby improving electrical characteristics of the manufactured device, and reduces soldering, thereby improving the reliability of the manufactured device. Moreover, the LTCC substrate minimizes a temperature coefficient of resonant frequency (Tf) by adjusting a thermal expansion coefficient, thereby controlling characteristics of a dielectric resonator.
The LTCC multilayer substrate is formed by forming circuits in a single ceramic substrate and vertically stacking a plurality of the ceramic substrates. Therefore, external terminals to be connected to the outside must be formed on an outer surface of the LTCC substrate and electrically connected to circuit patterns within the substrate.
FIGS. 1 and 2 show “a stack electronic component”, in which a laminated substrate having internal circuits is provided, via holes are vertically formed through the substrate, and external electrodes are formed by filling the via holes with a conductor, as disclosed by Japanese Patent Laid-open Publication No. Hei8-37251. As shown in FIGS. 1 and 2, via holes 7 are formed through a stack structure 5 and filled with conductors 9, and the conductors 9 within the via holes 7 are connected to the internal circuits. Then, through holes 10 are formed through the stack structure 5 and the conductors 9 are exposed to the through holes 10. The exposed conductors 9 serve as external electrodes 4 for electronic components. In this Japanese Publication, since the conductors 9 formed in the via holes 7 become the external electrodes 4, the external electrodes 4 have uniform dimensions and shapes and are easily formed.
However, the above Japanese Publication has a problem as follows.
The rectangular via holes 7 are simultaneously formed vertically through plural stacked green sheets by a punching method or etc. In this case, as shown in FIG. 3, the stacked green sheets are compressed in a direction of the punching by shear stress, and the internal metal patterns on the green sheets are not exposed in the via holes 7. The internal patterns must be exposed in the via holes 7 so as to be connected to the conductors 9 formed in the via holes 7 for serving as the external electrodes. However, the Japanese Publication as shown in FIGS. 1 and 2 does not solve the above-described problem.
There are various methods for forming external electrodes in the conventional low temperature co-fired ceramic multilayer substrate. First, as shown in FIG. 4, an internal pattern 2a is extended to a side of each ceramic sheet and exposed to the outside. Then, the ceramic multilayer substrate 3 is formed by stacking and firing the plural ceramic sheets at a high temperature.
An external electrode 4a is formed on an side surface of the ceramic multilayer substrate 3 by deposition without forming any through hole in the ceramic multilayer substrate 3 by the punching method. This method assures connection between the internal patterns and the external electrode. However, after the ceramic multilayer structure is cut into a plurality of unit ceramic multilayer substrates 3, the surface of the ceramic multilayer substrate 3 is ground so as to expose the internal patterns 2a prior to forming the external electrode 4a. Therefore, this method complicates a manufacturing process of the substrate and does not satisfy a requirement for mass production.
Further, FIG. 5 illustrates a further method for forming external electrodes. Herein, a notch being quarter-circular in shape is formed at a corner of each ceramic substrate so as to expose an internal pattern 2b, and an external electrode 4b is formed in each notch. Then, the ceramic multilayer substrate 3 is formed by stacking a plurality of the ceramic substrates, thereby integrating the external electrodes 4b into one external terminal. In this case, since the external electrodes 4b must be respectively formed on the ceramic substrates, the manufacturing process is very complicated. Further, since the dimensions of all the substrates are not uniform due to the difference of contraction ratios between individual substrates, the ceramic multilayer substrate is easily damaged by an external impact, or etc.
Moreover, FIG. 6 illustrates another method for forming external electrodes. Herein, a notch being quarter-circular in shape is formed at a corner of each sheet so as to expose an internal pattern 2c. Then, the ceramic multilayer substrate 3 is formed by stacking a plurality of ceramic sheets, and external electrodes 4c are simultaneously formed in the plural notches by the deposition. This method is generally used in forming external electrodes on a conventional low temperature co-fired ceramic multilayer substrate. As shown in FIG. 6, since the notches of the ceramic multilayer substrate 3 are not precisely aligned with each other, a material for forming the external electrodes is not uniformly deposited in every notch and the connection between the internal patterns and the external electrode becomes poor.
FIG. 7 illustrates yet another method for forming external electrodes, being similar to the method disclosed by Japanese Patent Laid-open Publication No. Hei8-37251. First, a plurality of ceramic green sheets are stacked vertically so as to form the ceramic multilayer substrate 3. Then, a notch is formed at a corner of the ceramic multilayer substrate 3 and an external electrode 4d is formed in the notch by the deposition. In this case, as described in FIG. 3, internal patterns 2d are not exposed in the notch in a step for forming the notch, thereby causing the same problem of not being connected to the external electrode 4d. 
Therefore, there is required in the art a method for simultaneously forming through holes on every sheet of a ceramic multilayer substrate by a punching method so as to simplify a manufacturing process of the ceramic multilayer substrate and improve the connection between the internal patterns and the external electrode.